Purification of fluids such as water and air are important objectives for public health, yet they pose ongoing challenges due to the need to balance of cost, convenience and effectiveness. As to cost, water and air are necessary for life, but populations at the lower end of the socioeconomic scale cannot support high retail costs for treated fluids, moreover purifier operating costs must be still lower to make them sustainable for either public or private providers. As to convenience, purifiers must require a minimum of maintenance, replacement and downtime. And as to effectiveness purifiers must provide essentially fail-proof service for long periods under a wide variety of operating conditions.
Uses of purification range from potable drinking water to fermentation media and separation of components in biological fluids. Likewise sanitizing purification of (re)circulated air in homes, offices, hospitals, clean rooms, air- and spacecraft are important application for filtration media. Thus air filtration products such as HEPA filters are popular to remove particulates such as dust, mold, allergens, and other material from the air. Among the numerous applications for materials that can remove and/or immobilize microbes, fermentation and other biotech processes are particularly important at present.
Existing water- and air purification methods are diverse, including distillation, reverse osmosis, ion-exchange, chemical adsorption, coagulation and filtering or retention (physical occlusion of particulates). Chemical methods include the use of reagents to oxidize, flocculate or precipitate impurities. The range of particle size exclusion depends on the size of pores or interstitial spaces in membranes and granular materials, respectively. Other methods use purification materials that react chemically with contaminants. Generally complete purification requires multiple complementary techniques, thus it is common to employ several devices in series, each with a different function. Illustrative of complementary methods are mixed resins to remove negatively and positively charged species, and charge-neutral species.
The need for extensive processing and special apparati add to the cost, energy inefficiency and technical sophistication of these methods. And the most economical techniques have been insufficiently effective against microbial contaminants such as bacteria and viruses. Membranes to remove components in the cellular size range are relatively costly, but the alternative is the use of strong oxidizers such as bleaches, halogens, reactive oxygen species such as ozone, and the like.
The minimum standards of the Environmental Protection Agency (EPA) for accepting antimicrobial water purification devices require a 6-log reduction at minimum (99.9999%) for common coliforms, represented by the bacteria E. coli and Klebsiella terrigena, for samples in which they are present at 1×107 (cells)/100 mL For devices for which common virus removal is claimed, as represented by process-resistant poliovirus 1 (LSc) and rotavirus (Wa or SA-11), the EPA's minimum standard for devices is a 4-log reduction, 99.99% of cells, from an 1×107 (cells)/L influent. Common cysts (protozoa), as represented by Giardia muris or Giardia lamblia, cause diarrhea, are difficult to treat medically, are widespread, and resist chemical disinfection. For devices that are claimed to remove cysts the EPA's minimum standard is a 3-log reduction, 99.9% of cysts removed, from 1×106 (cells)/L or 1×107 (cells)/L influent. The EPA has allowed the use of inanimate particles of comparable size to substitute for disease cells for purposes of testing devices to show satisfaction of these criteria.
Simple size exclusion and or aggressive oxidation can render fluids safe from microbes and organic toxins, but that is less true for dissolved inorganic toxins. Dissolved inorganic substances include metals, among which are the heavy metals. Thus for instance aluminum, arsenic ((V) and/or the more toxic (III)), copper iron, lead and zinc are commonly found in water, as is in some cases uranium. The uptake of these metals by filters is a function of charge state, pH, contact time and initial concentration. Common methods to remove them from, for example wastewater include chemical precipitation, membrane separation, osmosis, ion-exchange resins, solvent extraction, chemical redox reactions, coagulation and sorption; there is some overlap between these categories. Cost and efficiency are often key factors in choosing a purification protocol for this category of impurities. But in any case these impurities are an ongoing problem both because of their ubiquity and because they are subject to upper concentration limits that have been steadily reduced by regulatory and legislative bodies. Indeed the least amount that can be measured has been used in some rules as the threshold at which the presence of the metal is deemed excessive. Moreover there is commonly a need to remove metals from water due not to their toxicity but to their contribution to its hardness and the resulting unattractive and sometimes clogging deposits that they leave in their wake.
Thus there is an ongoing need for simple, inexpensive fluid purification and filtration methods and devices that can remove particulates, cells and dissolved inorganic species. There is a further need in the art for methods and devices that meet and significantly surpass the minimum EPA specifications for microbe-eliminating water purifiers suitable for consumer and/or industrial point-of-use applications.